Method for increasing endurance limit of steel articles



United States Patent 3,205,556 METHOD FOR INCREASING ENDURANCE LIMlT OFSTEEL ARTICLES John F. Romans, Berea, Ohio, assignor to EatonManufgfituring Company, Cleveland, Ohio, a corporation of No Drawing.Filed Sept. 10, 1962, Ser. No. 222,628 3 Claims. (Cl. 2990) Thisinvention relates, as indicated, to a method for improving certaincharacteristics of steel articles, and more particularly to a method forincreasing the strength and hardness characteristics of steel surfaces,particularly in spring steel surfaces, with a resultant increase in theendurance limit of the article.

Generally speaking, this invention depends upon the working of thesurface of a steel article by impinging against the surface thereof hardparticulate materials, such as steel shot. It has been known thatshot-peening of steel springs, for example, effects a cold working ofthe surface, and so alters the crystalline structure of the skin portionas to beneficially increase the endurance limit or fatiguecharacteristics against failures resulting from surface defects, such asnotches or scores which may result from prior operation. Such scores,notches, grooves and the like provide points of incipient prematurefatigue or fracture.

Early disclosures of the utilization of shot-peening as a means forimproving the characteristics of springs is found in the patent toVorwerk, 1,946,340, and the patent to Minich, 2,077,629. Both thesepatentees treated the entire surface by impinging small steel ballsagainst the surface and obtained improved characteristics, i.e. hardnessand resistance to fatigue failure.

From time to time, the prior art has made improvements upon the basicconcept of shot-peening. For example, it has been found that improvedresults are obtained when instead of indiscriminate blasting of both ofthe tension and compression sides of spring leaves, for instance, onlythe tension side should be so treated (Patent No. 2,252,823).

More recently, it has been found that the endurance characteristics ofsingle leaf springs can be improved by shot-peening the tension surfacewhile the spring is reverse stressed near to its yield point (Patent No.2,608,752). When peening is done under these conditions, the springtakes a considerable set during peening which must be allowed for whendesigning the quench form.

It has now been found that the amount of set can be greatly reduced bycarrying out the surface working operation at an elevated temperature,i.e. in the range of from about 200 F. to 600 F. without imposing thearticle to stress conditions, and consequently minimizing the extent ofset imposed on the article. So far as I am aware, shot peeningunstressed articles at such elevated temperatures is novel per se.

Moreover, certain articles are not easily adaptable to being shot-peenedunder stressed conditions, e.g. bearing mounts. Nevertheless, they alsorequire substantial improvement in fatigue characteristics for effectiveservice and are capable of being beneficiated in accordance with thepresent invention. This invention is especially adapted for use in themanufacture of passenger car rear suspension springs of the single ormultiple leaf types and having a predetermined contour or profile. It isalso well adapted for the manufacture of fiat springs having arectilinear profile or contour. A principal advantage of this process isthat contoured articles such as these do not undergo substantial changein contour or profile when shot-peened in accordance herewith.

Briefly stated, then, the present invention is in a method of improvingthe endurance characteristics of steel articles which comprisesadjusting the temperature of such article to Within the range from about200 F. to about 600 F. and shot-peening the article at such elevatedtemperature in the unstressed condition.

Shot-peening is an operation whereby hardened steel shot, preferablyaveraging about one thirty-second of an inch in diameter, are caused toimpinge upon the surface of the steel article being treated. In onedevice, the shot are thrown from the periphery of a wheel which isrunning at high speed, or they may be directed by a high velocity airstream against the surface of the article. In either case, the shot arecaused to travel at such a high velocity that, when they impinge uponthe surface of the article, they will remove any scale that is presentand also effect a working of the surface. Lineal velocities incommercial use range from about to about 350 feet per second. Uponcompletion of the shot blasting operation, the surface will have a veryfine-grained frosty or pebbly appearance. From 2 to 10 pounds of shotare impinged per square inch of surface being treated in a period oftime ranging from 10 to 60 seconds.

In order to illustrate the process of the present invention, a series oftest pieces of steel, SAE 5160 2.5" by 0.246 leaf type spring, 16" longhaving a Brinell hardness number of 430-444 were peened at roomtemperature and at elevated temperatures with cast steel shot having thecharacteristics hereinafter set forth.

All test springs were fiat and fashioned from the same bundle of steel,and processed at the same time to a Brinell hardness within the rangeabove stated. The tests were performed on a Pangborn Machine No. 4418.

As the fiat spring elements undergoing test left the tempering furnace,the surface temperature of each was measured with a Pyro surfacepyrometer. When the desired temperature shown in the data of Table I wasattained by spontaneous cooling, the spring element was fed into thepeening machine. In actual practice, the temperature .at which peeningis done may be attained by cooling from a tempering temperature which iswell in excess of the peening temperature range, or by elevating thetemperature of a pre-tempered article to within the desired range.

In the fatigue testing machine after the peening operation, the centralten inches of the test elements were placed under constant stress. Thetest plates were preset by placing each plate at the bottom of thestroke (maximum stress) before setting up with a Huggenberger tensometerfor an initial stress range of 30,000 to 160,000 p.s.i. Each piece wascycled to failure by fracture.

These tests show that peening under elevated temperatures withoutreverse stress causes substantially less change in a 16" chord heightfor the spring as a result of peening, than is found when peening isdone at ordinary temperatures but under stress.

The following table summarizes data obtained, the numerical values beingthe average of two test plates at each temperature value.

TABLE I Change in 16" Chord Height Due to Peening, inches AverageFatigue Lite Room Temperature 300 F.

It is anticipated that the effective temperature range will vary withthe composition of the steel being treated.

In all cases, the fatigue life of the steel spring element peened atroom and elevated temperatures was increased by a factor of from five totwenty times that of an unpeened spring element of the same material. Itwill also be observed that the amount of variation in the chord heightof a 16" long specimen in the case of the stresspeened material wasabout three times as great as the variation in the chord height due totemperature peening. Chord heights are measured at the mid-point of theelongated specimen supported by its extremities on a plane surface.

The shot analysis of the shot used in these tests is as set forth inTable II.

Thus, there has been provided an improved process for shot peening steelarticles, particularly articles subject to oscillatory motion andfailure by fatigue, e.g. springs of the leaf type, bearing mountings,etc., which process depends upon the utilization of elevatedtemperatures in the range of from about 200 F. to about 600 F. and shotpeening the articles at such temperature under normal shot peeningprocedures in the stress-free or unstressed condition.

Other modes of applying the principle of this inven tion may be employedinstead of those specifically set forth above, changes being made asregards the details herein disclosed provided the elements set forth inany of the following claims, or the equivalent of such be employed.

It is, therefore, particularly pointed out and distinctly claimed as theinvention:

1. The method of improving the endurance and set characteristics ofsteel articles which comprises in combination the steps of adjusting thetemperature of the article to within the range of from about 200 F. toabout 600 F. and shot-peening the article in the unstressed condition atsuch temperature.

2. The method of improving the endurance and set characteristics of leaftype steel spring elements which comprises in combination the steps ofadjusting the temperature of the spring element to within the range offrom about 200 F. to about 600 F. and shot peening the article in theunstressed condition at such temperature.

3. The method of improving the endurance and set characteristics, ofcontoured steel articles adapted to be subjected to oscillatory stresseswithout substantially changing said contour which comprises incombination the step of adjusting the temperature of the article towithin the range of from about 200 F. to about 600 F.,

and shot peening the article in the unstressed condition at suchtemperature. 7

References Cited by the Examiner .Corp., 1946, Mishawaka, Indiana, pages1 and 78.

RICHARD H. EANES, 111., Primary Examiner.

1. THE METHOD OF IMPROVING THE ENDURANCE AND SET CHARACTERISTICS OFSTEEL ARTICLES WHICH COMPRISES IN COMBINATION THE STEPS OF ADJUSTING THETEMPERATURE OF THE ARTICLE TO WITHIN THE RANGE OF FROM ABOUT 200* F. TOABOUT 600* F. AND SHOT-PEENING THE ARTICLE IN THE UNSTRESSED CONDITIONAT SUCH TEMPERATURE.